A 10-11 gas tank is very small even for a car so I am not sure what RV you have that would only have that size tank?
But, if it really is that small, yes you might have some challenges. It all depends, though, on where you are heading. In the lower 48 there are only a handful of places where gas stations are farther apart than 50 miles.
I would not carry any gas inside your RV. If you need to carry it you'll need to get some kind of exterior rack. Even if your RV says not to tow, you can probably still have a hitch mounted that would take a 1 1/2 or 2" bar and put a hitch carrier of some type. That wouldn't add that much weight and is not like actually towing something.
If you do this, get one of the higher-quality metal jerry cans (think of the kind you used to see on the back of jeeps) that are designed for this use, and they are safe enough....outside...